In a mobile telephone network, a user equipment may be in a region in which it is able to receive signals relating to more than one cell of the network. In order to determine which cell to connect to, measurements of received signal power of a signal from a base station (also known as an eNB in LTE standard) associated with each cell can be made and the cell corresponding to the base station having the highest or strongest Reference signal Received Power (RSRP) is chosen.
FIG. 1 shows a wireless communication network in which an eNB 12 of a macro cell, having a relatively high transmitted signal power and therefore covering a relatively large area 20, may be supplemented with one or more low power eNBs 14a, 14b, 14c corresponding to pico cells located within the area 20 covered by the macro cell. For example the low power eNBs may be used to provide extra capacity at a hotspot or to improve coverage in a low signal area within the area of a macro cell. This leads to user equipment 16 located near to the edge of a pico cell 14a receiving signals from both the macro eNB 12 and pico eNB 14a. In the RAN1#61Bis meeting in Dresden 28 Jun. to 2 Jul. 2010, it was proposed that an RSRP bias mechanism should be implemented such that user equipments should preferentially connect to the pico cell under certain circumstances.
This RSRP bias mechanism allows user equipments 16 being served by a macro cell 12 but near to a pico cell 14a, for example as determined by pathloss criteria, to camp on the pico cell 14a even if handover/cell reselection criteria based on RSRP are not met. According to the proposed biasing mechanism, an offset or threshold value is added to the RSRP of the low power cell before comparing it with the RSRP of the macro cell. Thus, the lower power cell will be selected even if the reference signal received power for that cell is lower than the RSRP for the macro cell by up to the threshold amount.
Without RSRP biasing, cell selection between the pico cell and the macro cell is based on the strongest signal, i.e. highest RSRP value. This leads to the footprint of the pico cell being limited by interference from the macro cell, and limits the number of user equipment devices that are able to benefit from the hotspot provided by the pico cell.
Using the RSRP bias mechanism, user equipment can be controlled to preferentially connect to the pico cell, even when the pico cell has a lower measured RSRP than the macro cell. In effect, this provides a range expansion for the low powered pico cell, increasing the size of the area served by the pico cell, and allowing more traffic to be offloaded from the macro cell to the pico cell. The RSRP biasing mechanism is applicable to heterogeneous networks comprising a mix of high power, macro, cells and lower power cells such as pico or hybrid cells.
However, while the use of RSRP bias has been proposed, details of how such a mechanism could be implemented have not yet been considered.